1. (Field of the Invention)
The present invention relates to a high voltage protecting circuit for avoiding any possible supply of an excessively high voltage to a cathode ray tube apparatus such as used in a television receiver set, computer display unit, etc.
2. (Description of the Prior Art)
In commercially available television receiver sets, it is well known that, when an excessively high voltage is applied to a cathode ray tube, the emission of X-rays increases to such an extend as to be hazardous to television viewers, and, therefore, a high voltage protecting circuit is generally utilized for avoiding any possible supply of an excessively high voltage to a cathode ray tube. An example of prior art high voltage protecting circuit is disclosed in the Japanese Laid-open Utility Model Publication No. 54-6824 and is illustrated in FIG. 2 of the accompanying drawings.
As shown in FIG. 2, the prior art high voltage protecting circuit comprises a power stabilizing circuit 6 from which an electric power is supplied to various circuits as subsequently described, and a horizontal oscillating circuit 7, a horizontal drive circuit 8 including a horizontal drive transistor 8a and a horizontal drive transformer 8b. The protecting circuit also comprises a horizontal output circuit 9 connected with the horizontal drive circuit 8 and including horizontal output transistor 9a, a damper diode 9b, an resonant capacitor 9c, a deflection coil assembly 9d and an S correcting capacitor 9e, a flyback transformer 10 operable to supply a high voltage to a cathode ray tube 11 and including a primary winding 10a, a secondary windings 10b and a third winding 12, a rectifying diode 13 connected with the third winding 12 of the flyback transformer 10, a smoothing capacitor 14, and a detecting circuit 16 connected with the rectifying diode 13 and including a detector output terminal 16a, a Zener diode 16b for providing a reference voltage and switching transistors 16c and 16d.
The prior art high voltage protecting circuit of the construction shown in and described with reference to FIG. 2 operates in the following manner.
A voltage generated from the third winding 12 of the flyback transformer 10 is rectified by the rectifying diode 13 and the smoothing capacitor 14, and the rectified voltage emerging from a rectifying circuit including the rectifying diode 13 and the smoothing capacitor 14 is then compared with the reference voltage generated across the Zener diode 16b in the detecting circuit 16. During a normal operation of the circuit, the rectified voltage is lower than the reference voltage, and therefore, no detector output is generated at the output terminal 16a of the detecting circuit 16. In this condition, the horizontal drive transistor 8a is operating normally.
However, in the event that the voltage generated from the flyback transformer 10 which is being supplied to the cathode ray tube 11 increases abruptly by some reason, the voltage generated from the third winding 12 of the flyback transformer 10 is correspondingly increased, accompanied by the increase of the rectified output voltage emerging from the rectifying circuit. Since this rectified voltage would be of a value higher than the reference voltage across the Zener diode 16b of the detecting circuit 16, the switching transistor 16c is rendered non-conducting and the switching transistor 16d is held in a conducting state. In this condition, the potential at the output terminal 16a of the detecting circuit 16 increases to hold the horizontal drive circuit 8 in an ON state, causing the horizontal output circuit 9 to cease its operation thereby to interrupt the supply of the high voltage to the cathode ray tube 11. In this way, the circuit acts in a protective mode.
Since according to the prior art high voltage protecting circuit the power source voltage is supplied at all time even during the protective mode of opertion, a non-loaded condition occurs when the horizontal output circuit 9 is brought to a halt. While the television receiver set is generally provided with the power stabilizing circuit, an output voltage from the power stabilizing circuit increases during the non-loaded condition to a value, for example, twice that during the normal operation particularly where the power stabilizing circuit is provided with a switching regulator as is well known to those skilled in the art.
In view of the foregoing, the prior art high voltage protecting circuit requires the use of various circuit component parts each having a high voltage breakdown characteristic, that is, resistant to a high voltage, or they have to be protected with the use of, for example, avalanche diodes. Whetherever the circuit component parts having the high voltage breakdown characteristic are used or whetherever the avalanche diodes are used, it tends to constitute a cause of the increased manufacturing cost. Moreover, the fact that the power source voltage is applied even though the television receiver set is turned off results in waste of energy.